In the semiconductor industry, lenses of the unit magnification catadioptric type are employed in the processing of semiconductive wafers to form integrated circuits. Very large scale integrated circuits are often fabricated by utilizing a precisely controlled stage to successively position adjacent regions, containing an integral number of individual microcircuits, on a semiconductive wafer with respect to an image (formed by such a projection lens) of a reticle containing a next level of the microcircuitry that is then printed on the semiconductive wafer at each of those regions. This step-and-repeat printing operation forms an array of adjacent regions of microcircuitry on the semiconductive wafer in rows and columns in an ordered parallel and orthogonal manner. Successive processing of the semiconductive wafer and printing of a further level of microcircuitry, aligned with the preceeding processed regions to a high (sub micron) accuracy, are typically employed in the fabrication of integrated circuits from the semiconductive wafer.
One problem peculiar to all optical projection lenses utilized in the processing of semiconductive wafers is the very shallow depth of focus they all possess. As a result each image must be focused before it is printed.
Because of the relatively large image sizes associated with unit magnification catadioptric lenses and the lack of surface planarity of the semiconductive wafers there is also a need to level the circuit side surface of the semiconductive wafer for each region to be exposed prior to the printing of the image of the reticle at that region so that a condition of best focus is obtained over the extent of the region.
In order to facilitate the focusing and leveling of the circuit side surface of the semiconductive wafer irrespective of the size of the wafer and the position of the region of the wafer being leveled, it would be highly desirable to employ a wafer chuck control system which provides vertical position, pitch and roll servo control of the wafer chuck, in decoupled response to inputs from a set of three focus detectors associated with the unit magnification catadioptric lens and positioned in locations surrounding the image of the reticle, to focus and level the plane of the circuit side of the semiconductive wafer prior to exposure. It is necessary to decouple the focus detector inputs from the vertical position, pitch and roll servo controller because, as the region position is varied, the location of each focus sensor relative to each vertical positioning element of the wafer chuck system varies as well. The loop gain associated with each focus detector vertical positioning element servo would change and could even invert by 180.degree. in direction as well, as will be explained hereinafter, if not suitably decoupled. Further, it is sometimes necessary to maintain the plane of the circuit side of the semiconductive wafer in the best focus condition possible with limited information, such as where the region being focused is positioned such that one or even two of the focus sensors are no longer directly above the wafer and provide no information at all.
In order to facilitate the precise positioning of adjacent areas of printed images without aligning each image to a previously printed and processed region on the semiconductive wafer, it would be highly desirable to be able to utilize the wafer chuck control system to maintain the plane of the circuit side surface of the wafer in a known position with respect to the precisely controlled stage as the wafer is leveled prior to exposure. This is especially true if the semiconductive wafer is to be processed in a mix-and-match manner where various types of alignment and exposure systems are utilized for different levels of microcircuitry and the exposed regions may be different for each type of alignment and exposure apparatus.
By way of example, consider the case where the step-and-repeat alignment and exposure system is used for printing the first layer and a scanning projection system, that is able to print the entire semiconductive wafer (with less resolution and accuracy but also with less cost), can then be used to print a later non-critical layer. Another example would be the case where another step-and-repeat alignment and exposure system is used for such a later non-crictical layer but it is desired to globally align the semiconductive wafer, by aligning only two locations, and shoot "blind" in order to save processing time and therefore reduce costs.
Unfortunately, however, many types of step-and-repeat alignment and exposure systems do not provide known position relationships between their wafer chucks and stages. By way of example, consider two prior art wafer chuck support systems as described in U.S. Pat. No. 4,383,757 entitled OPTICAL FOCUSING SYSTEM and issued May 17, 1983 to Edward H. Phillips and the aforementioned U.S. Pat. No. 4,391,494. Both patents illustrate wafer chuck support systems of the kinematically constrained type with the plane of kinematic constraint parallel to but separated a significant distance from the plane of the circuit side surface of the semiconductive wafer. This means that the region of the semiconductive wafer to be printed shifts laterally with respect to the stage as the pitch and roll attitudes of the wafer are modified according to the product of the separation distance and the sine of the differential pitch angle of the chuck. Because of the tolerance on wafer flatness, the magnitude of this product can be many microns (.mu.m) over the extent of the semiconductive wafer. Such lateral shift is not acceptable in light of the normal alignment figure of 0.1 micron(.mu.m).
The two machines disclosed in the cited patents deal with this problem differently. That of U.S. Pat. No. 4,383,757 does not provide for any wafer leveling once the step-and-repeat process for each semiconductive wafer has begun, while that of U.S. Pat. No. 4,391,494 does not allow any of the troublesome operation sequences, such as mix-and-match or shooting "blind", to be used whenever it is utilized for the photometric printing of the first level of microcircuitry on the semiconductive wafer.
Concomitantly, in order to facilitate an orthogonal relationship between the printed images and the rows and columns of the adjacent regions of microcircuitry of the semiconductive wafer and guarantee layer to layer angular-orientation overlay accuracy, it would also be highly desirable to employ an optical alignment system, including a stage reference sub-system, able to orient the image of the reticle precisely orthogonal to the co-ordinate axes of motion of the stage. This is true for all modes of multi-level semiconductive wafer processing even if all levels are to be printed with the same machine. This is because there is no guarantee that the level of microcircuitry present on each reticle will be angularly oriented precisely the same way with respect to the projection lens, either because that microcircuitry is slightly rotated with respect to the substrate upon which the reticle is formed or because the machine operator improperly mounts the reticle and it is slightly misoriented in its holder.
One prior art system that does have such an optical alignment capability is detailed at great length in U.S. Pat. No. 4,452,526 entitled STEP-AND-REPEAT PROJECTION ALIGNMENT AND EXPOSURE SYSTEM WITH AUXILIARY OPTICAL UNIT and issued June 5, 1984 to Karl-Heinz Johannsmeier and Edward H. Phillips and U.S. Pat. No. 4,473,293 entitled STEP-AND-REPEAT PROJECTION ALIGNMENT AND EXPOSURE SYSTEM and issued Sept. 25, 1984 to Edward H. Phillips. While this prior art system utilizes a projection lens of the reduction type, it does allow direct splitfield microscope viewing of the image of the reticle via a conjugate field forming a viewing port. A splitfield microscope is positioned adjacent to the conjugate field and views the image of the reticle thru the projection lens as it falls on a reference mark that is mounted on the stage. The splitfield microscope is coupled to a viewing binocular head which enables the operator to manually control the servo alignment of the image of the reticle's alignment windows with respect to the stage reference mark. The reticle is mounted on a three axis stage and the recticle stage is manipulated so that the images of the alignment windows are aligned on the reference mark.
The above structure is not practicable in conjunction with unit magnification catadioptric lens systems because of severe space limitations. In such systems it is extremely difficult to mount the reticle of a unit magnification catadioptric lens on a stage of any type, even a single axis rotational one, and indeed, the aforementioned U.S. Pat. No. 4,391,494 does not disclose such a stage. Neither does it disclose any direct viewing of the projected images nor any main stage reference apparatus. Therefore, printed images of that machine's reticle are randomly oriented and generally are not rotationally aligned with respect to the orthogonal axes of motion of its stage.
What is needed is a sub-system of the optical alignment system which can rotate and translate the orthogonal axes of motion of the stage so that the axes of motion of the stage are aligned with the actual position of the image of the reticle.
In order to minimize the time required to perform alignments of the previously printed and processed regions of the semiconductive wafer or the stage reference sub-system with the image of the alignment marks of the reticle, it would also be highly desirable to employ the optical alignment system to generate a two or three dimensional (as required for stage reference sub-system or wafer region alignment, respectively) offset signal proportional to the distance and indicative of the direction, required to move the stage to achieve alignment. The alignment system disclosed in the aforementioned U.S. Pat. No. 4,391,494 is non vectorial in concept and presents no directional information at all. Thus, mechanical displacement is required to generate alignment information and the resulting iterative alignment process is relatively slow in execution.
Finally, to enable the implementation of all of the uses for the optical alignment system, it is necessary to create a viewing port and miniature splitfield microscope for viewing the image of the reticle in a manner similar to that shown in the aforementioned U.S. Pat. No. 4,473,293, by providing a conjugate field, a novel method of microscope objective construction, and novel use of infinity correction in coupling said microscope objective to a utilization device.
Accordingly, it is the principal object of this invention to provide an improved step-and-repeat alignment and exposure system incorporating a projection lens of the unit magnification catadioptric type which allows direct viewing of an image of a first object, such as a reticle, and of a second object such as a stage reference sub-system or such as a semiconductive wafer.
Another object of this invention is to provide a unit magnification catadioptric lens incorporated in the step-and-repeat alignment and exposure system with a viewing port for observing an image plane of the lens.
Another object of this invention is to provide a step-and-repeat alignment and exposure system with a miniaturized microscope utilized in a splitfield manner and employing a novel method of microscope objective construction and use of infinity correction in coupling said microscope objective to a utilization device.
Another object of this invention is to provide the stage of the machine employing a step-and-repeat alignment and exposure system with a stage reference sub-system for providing an alignment reference for an image of the reticle by presenting a stage reference reticle.
Another object of this invention is to provide a step-and-repeat alignment and exposure system with an optical alignment system for generating a multidimensional offset signal proportional to the distance, and indicative of the direction, required to move the stage to achieve a selected alignment of the image of the reticle with either a stage reference mark image or alignment marks on a semiconductive wafer.
Another object of this invention is to provide a step-and-repeat alignment and exposure system with a sub-system for translating and rotating an orthogonal axes of motion of a main stage to achieve compatability with the actual position and orientation of the image of the reticle.
Another object of this invention is to provide a step-and-repeat alignment and exposure system with a sub-system for globally aligning the semiconductive wafer and shooting "blind".
Another object of this invention is to provide a step-and-repeat alignment and exposure system with a sub-system for aligning each previously processed region of the semiconductive wafer to the image of the reticle prior to photometrically printing the image of the reticle on the region.
Another object of this invention is to provide a method of utilizing the apparatus of the invention to calibrate the apparatus.
Another object of this invention is to provide a method of utilizing the calibrated apparatus of the invention to photometrically print first level semiconductive wafers.
Another object of this invention is to provide a method of utilizing the calibrated apparatus of the invention to photometrically print higher level semiconductive wafers.
Another object of this invention is to provide the main stage of a step-and-repeat alignment and exposure system with a wafer chuck system able to regulate the vertical position, pitch and roll of a plane of the circuit side surface of the semiconductive wafer in response to signals from a set of three focus sensors, while maintaining said surface in a known position with respect to the stage, and to provide a sub-system able to provide translational offsets to the stage coordinates along the axes of motion of the stage for maintaining the circuit side of the semiconductive wafer at the addressed stage co-ordinates during wafer region leveling.
Another object of this invention is to provide a stage of a step-and-repeat alignment and exposure system with a wafer chuck system able to maintain the plane of the circuit side of the semiconductive wafer in a controlled position with respect to the stage, and a sub-system adapted to manipulate the controlled position for maintaining precise registration of the circuit side surface of the semiconductive wafer at the addressed stage co-ordinates during wafer region leveling.
Another object of this invention is to provide a stage of the step-and-repeat alignment and exposure system with an extended function wafer chuck system and an electronic sub-system able to provide required rotational motion, for both global and regional semiconductive wafer alignment purposes, about an axis orthogonal to the co-ordinate axes of motion of the stage as well as maintaining precise registration of the center point of the wafer chuck at the addressed stage co-ordinates during both wafer region leveling and said rotational motion.
Still another object of this invention is to provide the wafer chuck system with a sub-system able to decouple a direct servo connection of the wafer chuck system to the set of three focus detectors associated with the unit magnification catadioptric lens and provide vertical position, pitch and roll servo control for achieving and maintaining semiconductive wafer region focusing and leveling irrespective of the size of the semiconductive wafer and the position of the region on the semiconductive wafer.
Still another object of this invention is to provide a method of decoupling the direct servo connection of the wafer chuck system to the set of three focus detectors.
These and other objects, which will become apparent from an inspection of the accompanying drawings and a reading of the associated description, are accomplished by the present invention comprising a main stage controlled for movement in a plane defined by first and second orthogonal axes; a wafer chuck for supporting the semiconductive wafer wherein said wafer chuck is supported on the main stage for rotational positioning about a third axis orthogonal to the first and second orthogonal axes; catadioptric projection lens means for imaging portions of a reticle onto the semiconductive wafer or onto a reference mark associated with the main stage, wherein an optical path is defined through the reticle and lens means; a light source for supplying illumination or exposure light; beam splitter means supplementing the catadioptric projection lens means and positioned along the optical path for viewing a projected conjugate image of the portions of the semiconductive wafer or reference mark which are illuminated by the projected image of the reticle; means for viewing selected portions of the projected conjugate image; and means for utilizing the viewed selected portions of the projected conjugate image.
More specifically, the above are accomplished according to the illustrated preferred embodiments of this invention by providing an improved step-and-repeat alignment and exposure system including a main stage controlled for movement to different positions along orthogonal X and Y axes; a wafer chuck mounted on the main stage and adapted for rotational movement about a third orthogonal Z axis for supporting a semiconductive wafer thereon; an optical sub-assembly mounted on the main stage for imaging a stage reference mark onto the plane of the upper surface, or circuit side, of the semiconductive wafer; a projection lens of the unit magnification catadioptric type for imaging illuminated portions of a reticle onto portions of the semiconductive wafer or the image of the stage reference mark, depending on the position to which the main stage is moved; a light source for directing uniform illumination or exposure light along an optical path extending thru the reticle and the projection lens; a beam splitter fashioned from a component prism of the projection lens for providing a viewing port at which a projected conjugate image of the selected portions of the semiconductive wafer or the image of the stage reference mark, illuminated by the projected image of the illuminated portions of the reticle, may be viewed; a pair of novelly constructed, infinity corrected microscope objectives adapted for viewing selected portions of the projected conjugate image; and coupled, thru a novel use of the infinity correction principal, to a pair of focusing lenses for re-imaging the viewed, selected portions of the projected conjugate image upon a pair of tv camera tubes.
The improved step-and-repeat alignment and exposure system also includes a wafer chuck focusing and leveling system adapted for regulating the vertical position, pitch and roll of the plane of the circuit side surface of the semiconductive wafer, in response to signals from a set of three focus detectors, with a combination of kinematic mounts, vertical drivers and position feedback sensors that together with a control surface in a known position with respect to the main stage and for providing translational offsets to the stage co-ordinates along the axes of motion of the stage for maintaining the circuit side of the semiconductive wafer at a set of addressed stage coordinates during wafer region leveling. In an alternative embodiment, a six degree of freedom support and position feedback sensor system together with an alternative control sub-system are adapted for maintaining the circuit side surface of the semiconductive wafer in precise registration with the set of addressed stage co-ordinates during region leveling.
In still another embodiment, the function of the six degree of freedom support and position sensor system, and the control sub-system is extended, for providing the rotational motion of the wafer chuck about the Z axis as well as maintaining the center of the wafer chuck in precise registration with the set of addressed stage co-ordinates.
The improved step-and-repeat alignment and exposure system also includes another sub-system able to provide a multi-dimensional offset signal proportional to the distance, and indicative of the direction, required to move the stage to achieve a selected alignment of the image of the reticle and either the stage reference mark image or alignment marks on the semiconductive wafer for minimizing stage alignment time; another sub-system able to rotate and translate the X,Y co-ordinate axes of motion of the stage into an offset and rotated U,V co-ordinate axes of motion of the stage for achieving compatability with the actual position and orientation of the image of the reticle; another sub-system able to provide selective decoupling of the set of three focus detectors from the wafer chuck's vertical position, pitch and roll servos for achieving and maintaining semiconductive wafer region focusing and leveling irrespective of the size of the wafer and the position of the region on the wafer; another sub-system able to provide global alignments and subsequent "blind" shooting of the semiconductive wafer for minimizing wafer processing time; and another sub-system able to provide regional alignments and immediate exposure of adjacent regions of a semiconductive wafer for minimizing alignment errors.